BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention relates to a process for the nitration of aromatic compounds. More
particularly, this invention relates to a process for the vapor phase nitration of
aromatic compounds susceptible of existing in the vapor phase at temperatures less
than about 190
0 C. in the presence of a molecular sieve catalyst.
[0002] Nitroaromatic compounds find use as solvents, explosives, dyes, perfumes, and analytical
reagents, and are important as intermediates in organic synthesis. As an example,
nitroaromatic compounds are convertible by reduction into primary amines, which, in
turn, are valuable intermediates in the synthesis of dyes, pharmaceuticals, photographic
developers, antioxidants, and gum inhibitors.
Descri tion of the Prior Art
[0003] Vapor phase nitration of aromatic compound is known in the art. The vapor phase nitration
of benzene and toluene at temperatures ranging from about 275° C. to about 310° C.
is described in McKee and Wilhelm, Industrial and En ineerin Chemistry, 28(6), 662-667
(1936) and U. S. Patent 2,109,873. McKee and Wilhelm catalyzed their reaction with
silica gel, with best results being reported by the use of 14 mesh material. Bauxite
and alumina were reported to be ineffective as catalysts in the vapor phase nitration
of benzene. More recently, U.S. Patent 4,107,220 described the vapor phase nitration
of chlorobenzene in the presence of molecular sieve catalysts having a pore size varyin
from about 5 A to about 10 A as a means for controlling the para-to-ortho isomer distribution
of nitrochlorobenzene. A suitable temperature range was reported to be from about
190° C. to about 290° C.
[0004] Although these prior art processes generally provide the desired product, they nevertheless
are limited in their applications. Principal among these limitations, which result
from the severe operating conditions, are low (aromatic compound) conversions and
contamination of the nitroaromatic compound produc- by undesirable by-products.
SUMMARY OF THE INVENTION
[0005] This invention is directed to a process for the vapor phase nitration of aromatic
compounds susceptible of existing in the vapor phase at temperatures less than about
190°C. in the presence of a molecular sieve catalyst. Accordingly, typical object:
of the invention are to provide a relatively low- temperature vapor phase process
for the preparation of nitroaromatic compounds and to provide a vapor phase nitration
process for converting aromatic compounds susceptible of existing in the vapor phase
at temperatures less than about 190° C. to the corresponding nitroaromatic compounds
characterized by high aromatic compound conversion and high nitroaromatic compound
selectivity.
[0006] These and other objects, aspects, and advantages of the invention will become apparent
to those skilled in the art from the accompanying description and claims.
[0007] The above objects are achieved by the process disclosed herein for the vapor phase
nitration of aromatic compounds susceptible of existing in the vapor phase at temperatures
less than about 190° C. which comprises contacting the aromatic compound with a nitrating
agent in the vapor phase in the presence of a molecular sieve catalyst at temperatures
between about 80° C. and about 190° C.
DESCRIPTION Of TNE PREFERRED EMBODIMENTS
[0008] In accordance with this invention, aromatic compounds susceptible of existing in
the vapor phase at temperatures less than 190
0 C- are nitrated in the vapor phase by a process which comprises contacting the aromatic
compound with a nitrating agent in the vapor phase in the presence of a molecular
sieve catalyst at a temperature between about 80° C. and about 190° C. The process
is characterized by high aromatic compound conversion and high nitroaromatic compound
selectivity..
[0009] Aromatic compounds su.itable for use in the present process are those susceptible
of existing in the vapor phase at temperatures less than about 190 C. (at atmospheric
pressures). Nonlimiting representatives of such aromatic compounds include aromatic
hydrocarbons, such as benzene, toluene, xylenes, ethylbenzene, cumene, and the like;.aromatic
ethers such as anisole, phenetole, and the like; and haloaromatic compounds such as
chlorobenzene, bromobenzene, iodobenzene, and the like. It has been found, however,
that the process of this invention is particularly efficacious with chlorobenzene
(also known as monochlorobenzene or simply MCB) and benzene.
[0010] The nitrating agents which are employed in the process of this invention are the
gaseous oxides of nitrogen higher than nitric oxide (NO) such as nitrogen dioxide
(NO
2), dinitrogen trioxide CN
2O
3), and dinitroge tetroxide (
N204)
. Of these nitrating agents, nitrogen dioxide is preferred. Thus, for convenience and
clarity the process will be described with reference to the preferred nitrogen dioxide
as the nitrating agent.
[0011] The molecular sieve catalyst employed in accordance with this invention are aluminosilicate
compounds having a well-defined crystalline structure; they may or may not be hydrated.
The fundamental structural units are silicon and aluminum atoms tetrahedrally coordinated
with four oxygen atoms. The silicate and aluminate units are generally joined to form
4- and 6-membered rings of oxygen atoms forming a simple and consistent arrangement
of polyhedra. Each polyhedr.a is a three-dimensional array of tetrahedra in a definite
geometric form.
[0012] The pore size of the molecular sieve catalyst suitable for use in the present process
is not narrowly critical. However, the pore size preferably should be at least 5 A.
The use of a substantially smaller pore size molecular sieve has been found to have
substantiall reduced effectiveness for producing the nitroaromatic compound under
conditions employed herein to produce the desired product. The maximum pore size of
the molecular sieve is not critical and is limited only in terms of practicality and
the availability of such molecular sieves.
[0013] Illustrative examples of the structure and synthesis of conventional molecular sieve
catalysts suitable for use in this invention can be had by reference to U.S. Patent
2,882,243; 2,882,244; 3,130,007; and 3,216,789; all of which are incorporated herein
by reference. For further information thereon and a general review of zeolite molecular
sieve catalysts, see Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York,
NY 1974.
[0014] Representative examples of suitable naturally occurring molecular sieve catalysts
are analcime, bika- taite, brewsterite, chabazite, clinoptilobite, bachiardite, edingtonite,
epistilbite, erionite, faujasite, ferrierite, gismondine, gmelinite, gonnardite, harmontome,
heulandite, kieselguhr, laumontite, ievynite, losod, mesolite, mordenite, natrolite,
omega, paulingite, phillipsite, sco- lecite, sodalite hydrate, stilbite, thomsonite,
yugawara- lite, and suitable synthetic zeolites are those zeolite compounds known
as "A", "N-A", "L
h", "P", "T", "X", "Y", "ZK-4", and "ZK-5".
[0015] Of these molecular sieve catalysts, mordenite is preferred. Some commercially available
mordenite molecular sieve catalysts are sold under the trademarks Zeolon 900 H (8-9
A pore size; Si0
2/A1
20
3 molar ratio -- 10/1) from Norton Company of Akron, Ohio, U.S.A.; and Zeolon 200 H
(8-9 A pore size; SiO
2/Al
2O
3 molar ratio -- 10/1) from Norton Company.
[0016] It will be appreciated that the invention is not limited to the aforesaid specific
molecular sieves and that other suitable molecular sieves having larger and smaller
pore size dimensions as well as larger and smaller SiO
2/Al
2O
3 molar ratios can be readily selected by persons skilled in the art in light of the
aforesaid disclosures and the specific illustrative examples.
[0017] In a preferred embodiment the molecular sieve catalyst is conditioned by pretreatment
with nitrogen dioxide at operating conditions to the saturation point (in the absence
of the aromatic compound). Pretreatment times in general range from about 1 minute
to about 1 hour or more. The actual pretreatment time, howeve will depend upon the
amount or quantity and pore structure of the molecular sieve catalyst, the flow of
the nitrogen dioxide, the operating conditions, and the like. Usually, pretreatment
for about 5 minutes about 30 minutes is sufficient.
[0018] The conditioning pretreatment, while not absolutely necessary, is preferred because
it permits almost immediate production of the nitroaromatic compc upon introduction
of the aromatic compound to the read In the absence of the pretreatment, measurable
nitroaromatic compound production is delayed until the mole ular sieve catalyst becomes
saturated with nitrogen dioxide.
[0019] The vapor phase nitration process of this ir vention is conducted at temperatures
between about 80° and about 190° C,, with temperatures between about 150° C. and about
175° C. being preferred in that at such preferred temperatures the rate of reaction
is reasonably rapid and little, if any, by-product formation occurs. It will be appreciated,
however, that tt particular temperatures employed for a given aromatic compound will
depend to some extent upon the boiling point or vaporization temperature of the particular
aromatic compound. For example, when chlorobenzene, which has a boiling point of 132°
C.., is the aromatic compound of choice, the vapor phase nitration is conveniently
carried out within the aforesaid preferred temperature range, with 175° C. being particularly
preferred. When benzene (b.p., 80° C.) is the aromat compound of choice, the vapor
phase nitration may be conducted at temperatures which encompass the entire operative
temperature range, that is, from about 80° C. to about 190° C'. Again, however, temperatures
between about 150°C. and about 175
0 C. are preferred.
[0020] The advantages accruing from conducting the vapor phase nitration of this invention
at the relatively low elevated temperatures between about 80° C. and about 190
0 C. include:
(a) greater selectivity to the desired nitroaromatic compound;
(b) little, if any, by-product formation (to contaminate the desired product);
(c) high material balance between reactants and products;
(d) lower energy requirements; and
(e) minimal thermal decomposition of the nitrogen dioxide.
[0021] The latter advantage [(e)] is particularly significant in that it, to a large extent,
influences the remaining advantages. It, of course, is well-known in the art that
at elevated temperatures nitrogen dioxide undergoes thermal decomposition into the
inert (for purposes of this invention) nitric oxide and molecular oxygen. The decomposition
begins at about 150° C. and is complete at about 620° C. The decomposition at various
temperatures is as follows:
Thus, at temperatures between about 80° C. and about 190
0 C., the maximum loss of active nitrogen dioxide by thermal decomposition into inert
nitric oxide is only about 5X, while at higher temperatures up to about 300° C., the
loss by thermal decomposition rapidly increases to 20-30% or more. Clearly, the magnitude
of the loss of nitrogen dioxide at temperatures higher th the operating temperatures
of this invention is wasteful and impractical. Moreover, if recirculation of the effluent
stream from such high temperature process is desired, it is necessary to employ an
additional step to reoxidize the inert nitric oxide to the active nitrogen dioxide
by treatment thereof with oxygen or an oxygen-containing gas such as air, with the
attends added cost and complexity. The additional cost and complexity of this added
step, however, is substantial reduced or eliminated altogether by the relatively low
temperature conditions employed in the process of this invention.
[0022] The vapor phase nitration process of this ir vention is carried out in the presence
of water, which is believed necessary to create and renew reaction sit on the molecular
sieve catalyst. The required water can be supplied by water of hydration in the molecular
sieve catalysts or, alternatively, by the separate addition of water via the feed
stream. When water of. hydration is present, no added water is required since once
the reaction is initiated, water produced during the course of the reaction (1 mole
of water for each 2 moles of nitroaromatic compound produced) is sufficient to sustain
it. If the molecular sieve catalyst is substantially anhydrous, it then becomes necessary
to add water in an amount sufficient to provide the required reaction sites. The separate
addition of water is usually preferred to ensure its presence in sufficient amounts.
The amount of water present, however, is not narrowly critical. Thus, amounts ranging
from nominal or trace amounts up to about 15% by volume of the feed stream are generally
sufficient, with amounts ranging from about 0.5% to about 5X by volume being desirably
used.
[0023] The vapor phase nitration of this invention is conveniently carried out in an apparatus
of the type suitable for carrying out chemical reactions in the vapor phase. It can
be conducted in a single reactor or in multiple reactors using either a fixed bed,
moving bed, or a fluidized bed system to effect contacting of-the reactants and the
mole.cular sieve catalyst. The reaction is generally carried out by continuously passing
a vaporous mixture of the aromatic compound and nitrogen dioxide over a bed of the
molecular sieve catalyst while maintaining a temperature between about 80° C- and
about 190° C., and usually, about 150° C. to about 175
0 C.
[0024] The reactant aromatic compound can be preheated to form a vapor which is then admixed
with gaseous nitrogen dioxide in a suitable reactor in predetermined relative proportions.
Vaporous aromatic compounds can be conveniently swept into the reactor at a constant
rate by a water-containing stream of carrier gas and thence admixed with a continuous
stream of nitrogen dioxide before contacting the heated catalyst bed. The reactants
can be charged into the reactor at any suitable flow rate.
[0025] As previously indicated, the reactant materials are conveniently swept into the reactor
by a stream of carrier gas. The carrier gas employed in the present process can be
oxygen or an oxygen-containing gas, for example, air, or an inert gas such as nitrogen,
helium, and the like. It is advantageous, however, to employ oxygen or an oxygen-containing
gas as the carrier gas due to the stoichiometry of the nitration reaction between
the aromatic compound and the nitrogen dioxide.
[0026] In the initial nitration reaction between the aromatic compound and the nitrogen
dioxide, it is believed that for each 2 moles of aromatic compound, 3 moles of nitrogen
dioxide are required to produce 2 moles of nitroaromatic compound, 1 mole of nitric
oxide, and 1 mole of water. In the absence of an oxyge source such as supplied by
the oxygen-containing carrie gas, the nitric oxide is lost, thereby reducing the nitrogen
dioxide selectivity to the nitroaromatic compound by at least 33X (one-third), as
well as the material balance between reactants and recovered products. In the presence
of oxygen (and the molecular sieve catalyst), however, the nitric oxide undergoes
the known reoxidation to nitrogen dioxide (stoichiometrically requiring 1 mole of
oxygen for each 2 moles of nitric oxide), which undergoes further reaction with additional
aromatic compound. Overall, therefore, little, if any, nitrogen dioxide is lost by
virtue of stoichiometrically produced nitric oxide.
[0027] The concentration of the aromatic compound in the feed mixture is not narrowly critical.
All that is necessary is that the concentration be sufficient to permit the reaction
to proceed at a reasonable rate. On the other hand, since the nitroaromatic compound
produced will have a high vaporization temperature (in fact, higher than the operating
temperature range, for example, nitrochlorobenzene isomers, b.p. 235-246
0 C.), the concentration should be such that the nitroaromatic compound produced will
not condense in the reactor. In addition, since mixtures of aromatic compounds and
air (the preferred aromatic compound carrier gas) are potentially flammable and explosive,
it is necessary, from a practical viewpoint, to operate at concentration outside the
flammable and explosive limits of the aromatic compound being employed. Generally,
concentrations between about 1% and about 15X by volume are desirably employed.
[0028] The relative proportions of reactants generally can range from about 1 to 5 moles
of nitrogen dioxide per mole of aromatic compound and, preferably, a ratio of about
2:1 to 3:1 is used.
[0029] The present process is suited to either batch or continuous operations. Continuous
operations can involve recirculation of the effluent stream unreacted aromatic compound
and nitrogen dioxide following isolation of the nitroaromatic compourrd product. Additional
reactants -- aromatic compound and nitrogen dioxide -- can then be charged to the
reactor along with the recirculated stream to continue the process in a subsequent
and continuous reaction. It will be noted that the substantial absence of side reactions
such as, for example, the thermal decomposition of nitrogen dioxide and undesired
by-product formation advantageously facilitates such continuous operations in that
extensive purification of the effluent stream is not required and, as previously noted,
the cost and complexity of reoxidation of the nitric oxide to nitrogen oxide to nitrogen
dioxide is substantially reduced or eliminated altogether.
[0030] The nitroaromatic compounds produced during the course of the vapor phase reaction
can be collected in a suitable chilled container, and purified by any appropriate
method and means known to the art such as, for example, distillation and crystallization.
Fractional crystallization in accordance with conventional procedures is especially
convenient for the separation of ortho- and para- isomers when a monosubstituted aromatic
compound having an ortho-para orientation substituent, such as chlorobenzene, is employed
as the reactant or starting material.
[0031] The recovered unreacted reactants, due to the substantial absence of side-reactions
to produce undesired by-products, are easily recycled to the reactor for further processing.
[0032] The following specific examples illustrating the best presently-Rnown methods of
practicing this invention are described in detail in order to facilitate a clear understanding
of tire invention. It should be understood, however, that the detailed expositions
of the application of the invention while indicating preferred embodiments, are given
by way of illustration only and are not to be construed as limiting the invention
since various changes and modifications within the spirit of the invention will become
apparent to those skilled in the art from this detailed description.
EXAMPLES 1 - 21
[0033] A stainless steel tube 40.64 cm (16 inches) in length and 2.54 cm (1 inch) outside
diameter, packed with a 35.56 cm (14 inch) bed of molecular sieve catalyst was employed
as the reactor. The catalyst, unless specified otherwise, was pretreated with nitrogen
dioxide at operating conditions (in the absence of the aromatic compounds) to the
saturation point, usually from about 5 minutes to about 1 hour.
[0034] A stream of aromatic compound was preheated and charged to the reactor tube in a
humidified or water-containing stream of air. The nitrating agent, nitrogen dioxide
unless otherwise specified, in a nitrogen carrier stream was mixed with the aromatic
compound/air stream shortly before contact with the heated catalyst.
[0035] The products were collected in a series of three chilled containers, the first of
which was chilled in an ice water bath and the second and third of which were chilled
in dry ice baths. Analyses were performed by gas chromotography on a a Varian Associates
Model 3700 instrument using a 1.83 meater (6 ft.) x 0.32 cm (0.125 inch) outside diameter
SP-1000, on 0.5% phosphoric acid treated Chrom. G Column prrogrammed from 90° C. to
210°C. at a program rate of 10° CC./minute.
[0036] The parameters a and the results are tabulated in Table 1.
[0038] Thus, it is apparent that there has been provided, in accordance with the present
invention, a process that fully satisfies the objects and advantages set forth hereinabove.
While the invention has been described with respect to various specific examples and
embodiments thereof, it is understood that the invention is not limited thereto and
that many alternatives, modifications, and variations will be apparent to those skilled
in the art in light of the foregoing description. Accordingly, it is intended to embrace
all such alternatives, modifications, and variations as fall within the spirit and
broad scope of the invention.
1. A process for the vapor phase nitration of aromatic compounds susceptible of existing
in the vapor phase at temperatures less than about 190° C., which process comprises
contacting the. aromatic compound with a nitrating agent in the vapor phase characte.
iz.ed.by carrying out the process in the presence of a molecular sieve catalyst at
a temperature between about 80° C. and about 190° C.
2. The process of Claim 1 characterized wher in the nitrating agent is nitrogen dioxide.
3. The process of Claim 1 characterized wher in the nitrating agent is admixed with
a carrier gas pri to reaction with the aromatic compound.
4. The process of Claim 3 characterized wher in the carrier gas is nitrogen.
5. The process of Claim 1 characterized wher in the molecular sieve catalyst is conditioned
by pretre ment with nitrating agent.
6. The process of Claim 5 characterized wher in the pretreatment is carried out for
about 1 minute to about 1 hour..
7. The process of Claim 1 characterized wher in the vapor phase reaction is carried
out at temperatur ranging from about 150° C. to about 1750 C.
8. The process of Claim 1 characterized wher in about 1 to about 5 moles of nitrating
agent are used per mole of aromatic compound.
9. The process of Claim 1 characterized wher in the aromatic compound is an aromatic'hydrocarbon.
10. The process of Claim 9 characterized wher in the aromatic hydrocarbon is selected
from the group c sisting of benzene and toluene.
11. The process of Claim 1 characterized wher in the aromatic compound is a haloaromatic
compound.
12. The process of Claim 11 characterized whe in the aromatic haloaromatic compound
is selected from t group consisting of chlorobenzene, bromobenzene, and iodobenzene.
13. The process of Claim 1 characterized wherein the concentration of the aromatic
compound in the feed mixture is between about 1% and about 15% by volume.
14. The process of Claim I characterized wherein the aromatic compound is admixed
with a carrier gas prior to reaction with the nitrating agent.
15. The process of Claim 14 characterized wherein the carrier gas is an oxygen-containing
gas.
16. The process of Claim 15 characterized wherein the oxygen-containing gas is air.
17. The process of Claim 1 characterized wherein water vapor is admixed with the feed
mixture prior to reaction between the aromatic compound and the nitrating agent.
18. The process of Claim 17 characterized wherein the water vapor is present in the
feed mixture in a concentration ranging from 0.1% up to about 15% by volume.
19. The process of Claim 1 characterized wherein the aromatic compound is chlorobenzene,
the nitrating agent is nitrogen dioxide, the molar ratio of nitrogen dioxide to chlorobenzene
is from about 1 to 5:1, and the temperature ranges from about 150° C. to about 175°C.
20. The process of Claim 1 characterized wherein the aromatic compound is benzene,
the nitrating agent is nitrogen dioxide, the molar ratio of nitrogen dioxide to benzene
is from about 1 to 5:1, and the temperature ranges from about 150° C. to about 175°
C.